Methionine adenosyltransferase (MAT) is a critical cellular enzyme that catalyzes the formation of S- adenosylmethionine (SAMe). In mammals, two different genes, MAT1A and MAT2A, encode for two homologous MAT catalytic subunits, cc1(MAT I and MAT III) and oc2(MAT II);while a third gene MAT2P, encodes for a regulatory subunit p that regulates MAT II. MAT1A is expressed mostly in liver while MAT2A is widely distributed. We showed in adult liver, increased expression of MAT2A is associated with rapid growth and de-differentiation, and the type of MAT expressed by the cell can influence the rate of cell growth. MAT1A expression correlates with slow growth and high SAMe levels, while the opposite occurs with MAT2A. During the past funding period we described transcriptional regulation of both MAT genes and elucidated some of the molecular mechanisms of MAT2A up-regulation in hepatocellular carcinoma (HCC). We developed in collaboration with Dr. Jose Mato the MAT1A knockout mouse model, which exhibits chronic liver SAMe deficiency, predisposition to liver injury, steatohepatitis, HCC, and delayed regeneration. These findings are highly relevant to human liver disease as MAT1A expression is decreased or absent in most patients with cirrhosis. We have also discovered highly novel actions of SAMe on growth and death response that are independent of its role as a methyl donor. The current proposal is focused on MAT genes and is based on several novel observations including 1) the ability of coding region methylation (rather than promoter region) to shut down MAT1A promoter activity, 2) stabilization of MAT2A mRNA with SAMe depletion, 3) ability of leptin to induce liver cell growth and MAT2A and MAT2|3 expression, 4) existence of two promoters in the MAT2P gene resulting in two variant proteins with possible different functions, and 5) potent growth modulatory effects of the MAT2P variants. The proposed studies are logical extension of these studies to: 1. examine transcriptional regulation of MAT1A - determine how coding region methylation regulates transcription and elucidate the molecular mechanism(s) of MAT1A down-regulation in HCC; 2[unreadable] examine transcriptional and post-transcriptional regulation of MAT2A - study MAT2A transcriptional regulation and how L-methionine withdrawal stabilizes the MAT2A mRNA; 3. examine transcriptional regulation of MAT2B and how it regulates growth - study MAT2p transcriptional regulation, define functional differences between the two variants and how they affect growth; and 4. examine the influence of MAT expression on tumorigenicitv. liver growth and HCC treatment - this will be done using in vitro and in vivo models including partial hepatectomy and HCC. Our overall goals are to better understand regulation and function of hepatic MAT genes, and to develop novel therapies against abnormal liver growth.